Aaaaaalright.... thanks for your input everyone so far, you're being very supportive! (is that the right word....?)
Sorry for the following wall of text, I really tried to make it as short as possible.
@Librarian: That was kind of what I initially envisioned, but since I know that the player already settled for Pyromancy, I'd think it futile to even have him make a Test on any other Discipline. But it will definitely be some kind of psychic ritual, and there will definitely be an emphasis on the different possibilities of solving a problem (i.e. Disciplines). I like the aspect of Chaos though, that's brilliant.
@Lynata: If you would have a mind to look up those Harrowing rules, that would be immensely helpful!
Acutally having him fail that test would not be that much of a problem. For various ingame reasons, my inquisitor is extremely protective of said psyker and would not under any circumstance have his life endangered, even if it means blemishing his own reputation (sounds weird, but makes sense in the chronicle). So even if he'd fail, I'd have the cavalry ride in and save his ass - that would even be conducive to my evil plans ^^
As for enforcing backstory on him, I did ask him beforehand and according to his total lack of reaction, he does not mind me surprising him, so I won't hold back.
So, from the input you've kindly given me so far I'd concoct something like this.
Silvanus undergoes the Tentatio Delectis, the Trial of Choosing. The point of that trial is to unlock the hidden potential within the Neonate, a source of power poetically called "that which is at bottom of one's heart". Since my psyker seems to lean towards Pryomancy, I'd guess the emotion that fuels his powers would be RAGE (not too uncommon in 40k philosophy...). That would actually add a nice contradiction to the character, for the player depicts him VERY composed and disciplined, almost inhumanly so.
So, to that end, he has to take part in a rite conducted by the elders of his coven - I have those already fleshed out - each one representing one Psychic Discipline. Probably intimidating and very much reminding of the suffering inflicted on him on Earth. He is required to ingest a potent hallucinogenic that puts him into a trance, loosely steered by the attendant Telepath. In the ensuing lucid dream he might be able to manifest a power of each Discipline and he might briefly brush with Chaos as well.
When the dream moves on, he will find himself on his home vessel, the Obsidian Edge, and encounter his Alter Ego, that forgotten part of him that dwells on the past and can't let go because Silvanus has completey supressed that memory and the pain that goes with it (that fits with the reoccurring subtheme of the chronicle, the "Enemy Within"). He will be forced to witness and relive what has transpired that fateful day he was exiled from his home, all the while being whispered to by the ghost that is him, who tries to coax from him the anger he hides so well by being REALLY mean and twisting the knife in the wound.
What could have happened? What could trigger a latent psyker's powers to emerge, preferably in a destructive fashion? Whatever it is, it will hopefully cause some raw nerves on my pyker turning the tension between him and the dream-self into hostility. Since the Alter Ego represents all the undealt with, the trauma inflicted on Silvanus by the awakening of his gift, it needs to be defeated, so let's escalate from emotional cruelty to "physical".
Hm... I guess I could let Silvanus fight with the temporary illusionary powers at his disposal, making the enemy really hard and wearing him down until he is almost defeated and frustrated enough to reach for that anger smoldering inside him, enabling him to consume his opponent in whitehot psi-flame... You know... "use your rage" XD
Well, that would be what comes to mind (it's quite late here, yawn), so if you have any comments/ideas, I'd be happy to hear them. Thx!